1. Field of the Invention
The invention relates to an optical amplifying apparatus which is used for a repeater station and an optical add/drop multiplexer in an optical communication system, and more particularly, to an optical amplifying apparatus and a complex optical amplifying apparatus which improve an optical signal to noise ratio by compensating loss as a function of wavelength of an optical transmission line. Further, the invention relates to an optical communication system comprising the apparatuses.
2. Description of the Related Art
Optical communication apparatuses with ultra-long distance and large capacity have been required with the objective of constructing a future multimedia network. Research and development on wavelength-division multiplexing (hereinafter abbreviated to “WDM”) is carried on in order to realize the large capacity, because it has the advantages of efficiently utilizing the properties of broadband/large-capacity of an optical fiber and the like.
An optical communication system which employs a WDM optical signal includes an optical transmitting station which generates the WDM optical signal with a plurality of optical signals whose wavelengths are different from each other being multiplexed therein and sends it out, an optical transmission line which transmits the WDM optical signal being sent out, and an optical receiving station which receives the transmitted WDM optical signal. Further, one or a plurality of repeater station having an optical amplifier is provided at some midpoint in the optical transmission line, in order to lengthen a transmission distance.
In the optical communication system, the transmission distance is limited by a gain balance based on a gain as a function of wavelength of the optical amplifier in the repeater station. The reason is that, when the repeater stations are cascaded in order to lengthen the transmission distance, gain inclinations which occur in the optical amplifiers in the respective repeater stations accumulate to degrade an optical signal to noise ratio (hereinafter abbreviated to “optical SNR”) in a channel with low signal power in the WDM optical signal, and to degrade a waveform in a channel with high signal power in the WDM optical signal due to a nonlinear optical effect or the like.
Therefore, in the conventional art, the optical amplifier is controlled so that fixed gains are provided in the all channels of the WDM optical signal, and the WDM optical signal is transmitted so that differences of signal powers between the respective channels are minimized.
Meanwhile, it is the recent tendency that the WDM optical signal increases its multiplicity and a wavelength band is broadened, in order to deal with an increase of a communication quantity.
There is loss as a function of wavelength in the optical transmission line such as an optical fiber, and when the wavelength band is broadened, a loss difference between a channel in a short wavelength side and a channel in a long wavelength side in the WDM optical signal becomes larger as compared with the conventional art.
For example, as to the loss as a function of wavelength of the optical fiber in 1550 nm band, it is known that a loss coefficient is about −0.0005 dB/nm/km. Accordingly, when a wavelength bandwidth of the WDM optical signal is 25 nm and the WDM optical signal is transmitted for a distance of 100 km, loss in a longest wavelength side becomes larger than loss in a shortest wavelength side by approximately 1.25 dB.
Especially, as to the loss as a function of wavelength of the optical fiber in excess of 1600 nm band, it becomes larger than −0.0005 dB/nm/km, and hence the difference between the loss in the longest wavelength side and the loss in the shortest wavelength side becomes still larger.
Moreover, it is known that a stimulated Raman scattering which is one of nonlinear optical phenomena occurs in the light propagating through the optical transmission line such as the optical fiber.
The stimulated Raman scattering transfers signal power in the short wavelength side to the long wavelength side due to an interaction with optical phonon in the optical transmission line, so that signal powers between the respective channels become nonuniform to cause differences of the signal powers between the channels.
Since the signal power which is lost from the channel with the short wavelength due to the stimulated Raman scattering depends on the number of multiplexed wavelengths, channel spacing, the transmission distance and the like, the difference between the signal power of the channel in the longest wavelength side and the signal power of the channel in the shortest wavelength side becomes still larger by broadening the wavelength band or lengthening the transmission distance.
For example, when the channel spacing, the number of multiplexed wavelengths (channel number), the transmission distance and transmitting signal power per channel of the WDM optical signal are respectively set at 100 GHz, 32 channels, 100 km, +5 dBm/ch, a difference in the signal power due to the stimulated Raman scattering between the channel in the longest wavelength side and the channel in the shortest wavelength side is approximately equal to 1 dB.
It should be mentioned that, even when the WDM optical signal is transmitted by using the optical amplifier which is adjusted to have a minimum wavelength dependency of a gain, in the widened wavelength band of the WDM optical signal and the lengthened transmission distance, there occurs a difference in signal power between the channel in the longest wavelength side and the channel in the shortest wavelength side, because of the aforesaid loss as a function of wavelength and the stimulated Raman scattering in the optical transmission line. The difference leads to another difference in the optical SNR between the channel in the longest wavelength side and the channel in the shortest wavelength side, thereby limiting the transmission distance.
Hence, it is an object of the present invention to provide the optical amplifying apparatus and the complex optical amplifying apparatus which compensate a loss as a function of wavelength of the optical transmission line by a gain as a function of wavelength.
It is another object of the present invention to provide the optical communication system capable of broadening the wavelength band and lengthening the transmission distance, by using, as the optical amplifier in the repeater station, the optical amplifying apparatus and the complex optical amplifying apparatus which compensate a loss as a function of wavelength of the optical transmission line by a gain as a function of wavelength.